As the use of digital data for transporting communications signals continues to grow in the consumer sector, more and more homes and offices are beginning to receive telephony services using an Internet connection. Furthermore, the physical network that transports the data may comprise a community antenna television (“CATV”) coaxial cable (“coax”) network, or other high-speed network, including digital subscriber line (“DSL”). In such a scenario, a drop from a service provider's outdoor coaxial cable plant connects to subscriber premise equipment (“SPE”), which is sometimes located outside a home or small office, or inside the premises. Within the SPE, television video signals and data signals, such as for example, Internet signals, are broken out and routed to their corresponding equipment. A cable modem is an example of an SPE with a coaxial cable connection for interfacing with a CATV network and typically an Ethernet or USB connection for providing the data signal. To the cable modem, a multimedia terminal adaptor (“MTA”) is typically connected if a customer receives telephony services via the data signal. In addition, some manufacturers house the cable modem and MTA as a single device, for example, a Touchstone™ Telephony Modem product (“TTM”), as offered by ARRIS International, Inc.
A TTM provides a user with telephony services over an Internet data network, namely, a cable modem termination system (“CMTS”) network. Cable telephony generally, the technology of which is known in the art, provides a subscriber with telephony service that is transparent as to the source of the service. In other words, a user plugs a telephone into a TTM and, in addition to basic telephony connectivity, can access a set of features such as, for example, dial tone, call waiting and other features similar to those provided by a traditional plain old telephone system (“POTS”).
When a TTM is installed in a consumers home or office, either by the consumer or by an installer working for a service provider, the TTM may be connected to the incoming CATV coax and then an output telephony wire from the TTM may be plugged in to a telephone wall jack, such as, for example, an RJ-11 jack. When the output telephone wire from the TTM is connected to the wall jack, service available on the wire is available to all of a plurality of telephone jacks in the home or office, as the plurality of wall jacks that provide service to a given customer are typically all connected in parallel. The output of the TTM typically provides a battery line voltage signal that is similar to the battery voltage provided on a traditional POTS line, as known in the art. Thus, a subscriber line interface circuit (“SLIC”), which is known in the art, inside the TTM converts incoming digital data and outputs it as an analog signal that mimics the signal provided by a POTS line card, including the battery voltage signal. Accordingly, the signals applied to all the plurality of telephone jacks connected in parallel appear to be traditional POTS, signals.
However, if an existing telephony service connection is not disconnected before the TTM is connected to the plurality of telephone jacks, damage may occur to the SLIC as well as the existing telephone company's line card, because the line card and the SLIC are both producing a battery voltage on the tip-ring twisted pair of wires inside the house or office. The nominal battery voltage for POTS systems is 48V DC, however, the actual battery voltage may range from between 42 V to 56 V. Accordingly, if either the SLIC or the line card are not outputting battery voltage at exactly the same voltage, or they are not both connected to the same ground reference potential, then the battery supply voltage of one may try to regulate itself based on the other's battery voltage. Thus, battery voltage supply of the lower voltage may draw high current in attempting to raise the battery voltage on the line to the other voltage, or at least the voltage at which it is designed to operate.
Although SLICs and line cards may typically use overcurrent protection devices, such as timed relays, to prevent circuit damage, service may be interrupted which a relay switch is open, and eventually, as the protection device cycles through repeated on-off cycles while the battery voltage differs from its design voltage, the protection device itself may be destroyed. While a user may be aware that their telephone is not working, or their newly purchased TTM is making strange noises and is getting warm, they may not know the cause of this irregular operation.
Thus, there is a need in the art for a method and system that detects when more than one battery voltage is being supplied to a telephone wire network within a house or office, and provides an alarm to alert a user or installer so that appropriate measures, such as for example, disconnecting the internal wiring of the house from the telephone company's wiring network, can be performed.